1. Field of the Invention
The invention relates to a magneto-optical read/write device for reading and writing information by magneto-optical means on a magneto-optical record carrier, the device comprising a magneto-optical read/write head which includes a radiation source for generating a radiation beam, and an optical system for focusing the radiation beam on a magneto-optical detection system via the record carrier. The Detection includes a first and a second radiation-sensitive detector, an output of the first detector being coupled to a first input of an adder circuit and a first input of a subtracter circuit, and an output of the second detector being coupled to a second input of the adder circuit and a second input of the subtracter circuit. The output of the adder circuit is coupled to a first input of a selection circuit, the output of the subtracter circuit is coupled to a second input of the selection circuit, and the output of the selection circuit is coupled to a signal processing circuit.
2. Description of the Related Art
A device of this type is known, for example, from U.S. Pat. No. 4,833,662, and device is intended to be used in combination with magneto-optical record carriers whose tracks are subdivided into sectors in which are areas preceded by sector headers in the form of pre-arranged pit structures.
Such pit structures produce intensity modulation in the radiation beam. The sector header information may be recovered on the basis of the output signal of the adder circuit. The information in the areas is recorded in the form of magnetic domains. The information represented by these domains can be recovered from the output signal of the subtracter circuit. Depending on the scanned type of domain structure the selection circuit passes on either the output signal of the subtracter circuit or the output signal of the adder circuit to the signal processing circuit.
The detection signals produced by the detectors have a large DC component. This means that the sum of the detection signals also has a large DC component, whereas the difference between the detection signals has a much smaller DC component. Since both the sum of detection signals and the difference between the detection signals are to be processed by the same signal processing circuit, it is important that the DC component at the output of the selection circuit should not depend on the particular signal.
In prior art magneto-optical device this is achieved by coupling the output of the detectors to the inputs of the adder circuit and subtracter circuit by an AC coupling. The adder circuit and the subtracter circuit are then exclusively supplied with signals free from DC current, due to which the DC components on the output of the subtracter circuit and on the output of the adder circuit are equal.
Such solution, however, causes problems when information is recorded and erased. During recording, the intensity of the radiation beam is switched between a high write level and a low read level in accordance with the information to be recorded. During scanning of the sector headers, the intensity of the radiation beam is constantly maintained at the read level. This means that the mean intensity of the radiation beam during the writing is higher than the mean intensity of the radiation beam during the scanning of the sector headers. Such a variation of the DC level is also found during erasure, whereas the intensity of the radiation beam when the sector headers are scanned is equal to the read level, and the intensity when areas between the sector headers are scanned is set to a high write level. The differences between the mean intensities of the radiation beam lead to the fact that the DC level of the detection signals during recording or erasure is higher than when the sector headers are scanned. When a change is made from information recording to sector header reading, a jump-like change occurs in the detection signals.
When AC coupling between the detectors and the adder circuit is applied, this means that the output signal of the adder circuit presents an exponential transition phenomenon on which the detection signal proper is superposed. For, the voltage across the AC couplings will change at a rate determined by the time constant of the coupling. Such a detection signal superposed on an exponential transition phenomenon causes problems with the information recovery. On the one hand, the use of very small time constants is undesired because this leads to detection problems when coded signals not free from DC current are read, such as, for example, the frequently used (2,7) code. Large time constants are also undesired because of their attendant long duration of the transition phenomenon. The problem of transition phenomena occurring in recording devices may be eliminated by utilizing what is commonly referred to as squelch circuits which short-circuit the signal for a brief period of time. However, this always leads to compromises as regards the performance of the device.